The Rhineland is the area of Germany between France and the river Rhine
If Germany were to attack France, they would most likely move their forces through the Rhineland
After WWI, the Rhineland was made a demilitarised zone as part of the Treaty of Versailles and confirmed by the Locarno Treaty of 1925 to protect France from future German attacks
The demilitarisation of the Rhineland was a clear breach of the Treaty of Versailles and the Treaty of Locarno, justifying Britain and France in imposing sanctions or taking military action against Germany
League of Nations condemned the action on the 14th of March, 1 week after the event
France condemned the reoccupation of the Rhineland and sought Britain's support in military action
Britain issued a formal protest to Germany and attempted to avoid military action through talks with France
Reasons why Britain didn't want to take action:
Strong pacifist movement in the 1930s, with many opposing war as wasteful and costly
British public indifference towards German troops occupying their own territory
Belief among the British public that Germany had "genuine grievances"